Plate heat exchangers are well known devices for the transfer of heat between two different media, in particular fluids. Plate heat exchangers usually comprise a plurality of heat exchanger plates, wherein each heat exchanger plate comprises a pattern of indentations as well as inlets and outlets for the two media. Each pair of neighboring plates is joined in such a way that channels for the transport of the separate media are created. The two media will then be allowed to circulate between alternating pairs of plates to allow a transfer of heat through the heat exchanger plates. The pattern of indentations of one plate will be in contact with the indentation patterns of the two neighboring plates. This way the plates are kept slightly spaced and the shape of the fluid paths can be adjusted to improve the efficiency of the heat exchange.
In the state of the art, it is common to use a so called herringbone pattern of indentations comprising ridges and valleys that force the flow of the media to accelerate and decelerate repeatedly within the plane of the heat exchanger plate. This usually leads to a large variation of the flow rate of the fluids which reduces the effectiveness of the heat transfer. Thus, a pattern of indentation that allows for a more homogeneous flow of the fluids would be beneficial.
There are furthermore two important types of heat exchangers known in the state of the art, namely brazed heat exchangers and gasketed heat exchangers. Since the fluids in the heat exchanger will usually be provided under a large pressure, one needs to ensure that the plates of the heat exchanger are held firmly together. In a brazed heat exchanger each two neighboring heat exchanger plates are brazed together where the indentation patterns meet. On the other hand, in a gasketed heat exchanger the plates are kept under tension by external forces, for example by introducing bolds through bores of the plates. Consequently, in a gasketed heat exchanger the heat exchanger plates are kept under a pretension.
In order to improve the efficiency of the heat exchange, it has been tried to reduce the surface area used as contact surface of the neighboring heat exchanger plates or to reduce the thickness of the heat exchanger plates.
In U.S. Pat. No. 8,091,619 B2 a heat exchanger of the type mentioned above is disclosed. Therein the herringbone pattern of indentations is replaced by a plurality of dimples, comprising tops and bottoms. The flat tops of one plate are brazed together with the flat bottoms of a neighboring plate. Thus, the stability of such a brazed heat exchanger can be improved allowing to reduce the thickness of the heat exchanger plates. At the same time the surface area at which each two neighboring plates meet is optimized. Thus, the efficiency of such a brazed heat exchanger is improved.
In case of a gasketed heat exchanger such a construction may be problematic. Gasketed heat exchangers have the additional problem of plastic deformation at the contact areas of the heat exchanger plates. Such deformations occur partly due to the heat exchanger plates being kept under a pre-tension, and due to the relative pressure difference of the fluids. This may result in plastic deformations at the contact areas of the heat exchanger plates where such plastic deformations may form a bypass for the fluids especially if the relative pressures of the fluids changes, resulting in a lower performance of the heat exchanger.